The present invention relates to a membrane and to a sensor (particularly but not exclusively a biosensor) incorporating such a membrane.
Various sensors are known which incorporate a membrane permeable by diffusion to an analyte species of interest whereby the species may be directly or indirectly detected by a detection arrangement provided at one side of the membrane. The detection arrangement may be an electrochemical detection arrangement (e.g. an enzyme electrode). Examples of such sensors are biosensors incorporating an enzyme electrode whereof the enzyme is capable of interaction with the selected analyte to produce a change which may be determined by the electrode arrangement as an indirect representation of the presence or amount of the analyte. By way of example, there may be mentioned a glucose biosensor based on the enzyme glucose oxidase which catalyses the oxidation of glucose by molecular oxygen to gluconic acid and hydrogen peroxide. The increased in hydrogen peroxide concentration or decrease in oxygen concentration may be determined by an amperometric electrode.
Such glucose biosensors may be used for the determination of glucose in either blood or fruit juice.
A problem does however exist with sensors based on electrochemical detection systems in the case where the sample being analysed contains ascorbate species in addition to the particular analyte of interest since ascorbate is an interferent in the electrochemical detection. It will thus be appreciated that ascorbate is an interferent in the determination of glucose in fruit juice.
A solution to the problem of ascorbate interferents is to use an ascorbate rejecting membrane between the electrochemical detection system and the sample being analysed. However the known membranes have the disadvantage that they only reject ascorbate over a relatively limited pH range and are not effective at the acidic pH values of fruit juice. Thus for example, unmodified cellulose acetate becomes ineffective around the pKa of ascorbate (pH 4.1) and is only really effective above ca pH6.
It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantage.
According to a first aspect of the present invention there is provided a membrane for use in a sensor having a detection arrangement said membrane being permeable by diffusion to an analyte species of interest to tie detected directly or indirectly by the detection arrangement wherein said membrane is of a cellulosic material incorporating a hydrophobic agent.
By direct detection we mean that the permeating species is itself detected. By indirect detection we mean that the permeating species after passing through the membrane, interacts with a further component to generate the actual species which is detected.
According to the present invention there is provided a sensor comprising an electrochemical detection arrangement and a membrane in accordance with the first aspect of the invention positioned in use of the sensor between said arrangement and the sample being analysed.
We have found that cellulosic membranes incorporating hydrophobic agents are capable of ascorbate rejection over a wide range of pH values and are therefore useful in the production of sensors which have an electrochemical detection system and which are used in applications where avoidance of ascorbate interference is required. The membranes of the invention have the advantage that they are also wettable, a property which is derived from the Cellulosic nature of the membrane
The cellulosic material may, for example, be cellulose, cellulose nitrate or a cellulose ester such as cellulose acetate or cellulose butyrate. Most preferably the membrane is of cellulose acetate. For preference the cellulose acetate has an acetate content of 35% to 45% by weight. e.g. about 40% by weight.
The amount of hydrophobic agent incorporated in the membrane is preferably 1 to 90% by weight (based on the total weight of the membrane). More preferably this amount is 1% to 50%, even more preferably 10% to 50%, and most preferably 14% to 45%. Thus, for example, the membrane may comprise 14% to 30%. more specifically 14% to 20% by weight of the hydrophobic agent.
The hydrophobic agent may, for example, be a substantially water immiscible fatty or oily substance, for example a lipid or ester.
Preferred hydrophobic esters are lower alkyl (e.g C1-C1) esters of long chain aliphatic acids (e.g. having a chain length of up to 18 carbon atoms). A particularly preferred ester is iso-propyl myristate. Other esters which may be used are esters of organic acids, e.g. dioctyl phthalate, as well as esters such as dioctyl adipate. A further hydrophobic agent which may be used is diphenyl ether.
A sensor in accordance with the invention may, for example, be a biosensor comprising an enzyme layer which is capable of interacting with an analyte species of interest to produce an electrochemically detectable change, an electrochemical detection system on one side of the layer for detecting said change, and a membrane in accordance with the first aspect of the invention provided either between the enzyme layer and the detection arrangement or on the side of the enzyme layer remote from the detection arrangement.
The membrane may have a thickness of 0.1 to 200 microns, preferably 4 to 50 microns.
Membranes in accordance with the invention may be produced by conventional casting techniques in which a solution of the cellulosic material and the requisite amount of the ester in a volatile solvent is cast usually onto a flat surface and the solvent allowed to evaporate. Alternatively it is possible to employ a xe2x80x9cspin castingxe2x80x9d technique in which the membrane is produced by applying a solution (of the type defined in the previous sentence) to a flat surface which is then rotated (usually about a vertical axis) at a speed which causes the solution to be evenly distributed and the solvent to be evaporated so as to produce a membrane of uniform thickness.
The enzyme layer may be provided using conventional techniques, e.g. by immobilising the enzyme in a cross-linked glutaraldehyde matrix. If desired the enzyme layer may be supported on a highly permeable membrane (e.g. a dialysis membrane) or between two such membranes.
The membrane can be used as a diffusion restricting membrane and is therefore advantageously used as the outer membrane combining properties of diffusion restriction and pH independent ascorbate rejection. Furthermore, in the case of glucose sensors, the use of the membrane confers extreme linearity for glucose in addition to the properties outlined in the previous sentence. Since the membrane has diffusion restricting and glucose linearising properties it is possible to use the membrane of the invention in sensors having a detection arrangement other than an electrochemical detection arrangement.
It is also possible for the membrane to be used as an inner membrane of a sensor, i.e. between an enzyme layer and the detection arrangement.